Strain rate dependence of the work of fracture response of an amorphous poly(ethylene-naphthalate) (PEN) film

Abstract

The essential work of fracture (EWF) response of an amorphous film of high molecular weight poly(ethylene-2, 6-naphthalate) (PEN) was determined on tensileloaded deeply double-edge notched (DDEN-T) specimens at various deformation rates (v=1, 10 and 100 mm/min) at ambient temperature. The DDEN-T specimens showed full ligament yielding (marked by a load drop in the force-displacement curves) based on which the yielding was separated from the subsequent necking. The yielding related specific essential work of fracture (We,y) changed parallel with the yield strength (σy), whereas the critical crack tip opening displacement (ϵy, 0) remained practically constant in the v range studied. As a consequence, We,y(v) could well be estimated by the product of σy(v).ϵy,0. Necking occurred by cold drawing superimposed by some strain-induced crystallization at the highest deformation rate. The necking-related EWF terms strongly depended on the deformation (strain) rate. Based on previous results on amorphous PENs of various molecular weight (MW), it was argued that increasing deformation rate corresponds to decreasing MW and vice versa. This was reasoned by assuming that only a part of the entangled molecular chain participates in the load distribution under high strain rate conditions.